No abstract

A sediment mass balance constructed for a 16-km reach of the Snake River downstream from Jackson Lake Dam (JLD) indicates that river regulation has reduced the magnitude of sediment mass balance deficit that would naturally exist in the absence of the dam. The sediment budget was constructed from calibrated bed load transport relations, which were used to model sediment flux into and through the study reach. Calibration of the transport relations was based on bed load transport data collected over a wide range of flows on the Snake River and its two major tributaries within the study area in 2006 and 2007. Comparison of actual flows with unregulated flows for the period since 1957 shows that operations of JLD have reduced annual peak flows and increased late summer flows. Painted tracer stones placed at five locations during the 2005 spring flood demonstrate that despite the reduction in flood magnitudes, common floods are capable of mobilizing the bed material. The sediment mass balance demonstrates that more sediment exits the study reach than is being supplied by tributaries. However, the volume of sediment exported using estimated unregulated hydrology indicates that the magnitude of the deficit would be greater in the absence of JLD. Calculations suggest that the Snake River was not in equilibrium before construction of JLD, but was naturally in sediment deficit. The conclusion that impoundment lessened a natural sediment deficit condition rather than causing sediment surplus could not have been predicted in the absence of sediment transport data, and highlights the value of transport data and calculation of sediment mass balance in informing dam operations.

Channel and floodplain form are primarily determined by the flux of water through a reach and the associated transport of the sediment delivered from the upstream watershed. The dominant paradigm of fluvial geomorphology is that the size of the bankfull channel and the characteristics of the adjacent floodplain are maintained by the current hydrologic and sediment supply regimes (Wolman and Miller, 1960; Andrews, 1980; Leopold, 1994). The linkage among flow regime, sediment supply, and channel and floodplain form is well illustrated on regulated rivers where the flow regime and sediment supply are altered by dams. Bed incision under conditions of sediment deficit has been widely described (Mostafa, 1957; Komura and Simmons, 1967; Galay, 1983; Williams and Wolman, 1984). The longitudinal change from near-dam sediment deficit to sediment surplus further downstream has been described by Andrews (1984) and Grant et al. (2003), although local differences in bed texture, channel organization, and valley confinement affect the magnitude of channel and floodplain change in any specific reach (Lagasse, 1981; Grams and Schmidt, 2002, 2005).